1. Field of the Invention
The instant relates to methods of and apparatus for levitating an eddy current probe. More particularly, the instant invention is directed to methods of and apparatus for levitating an eddy current probe wherein the probe is presented to specimens with a stand-off distance that is precise, constant and highly reproducible.
2. Technical Considerations and Prior Art
In photomicrolithography it is well-known to use eddy current technology to measure and control precision thicknesses of metallic film, vapour deposited in layers measured in the range of tens of microinches. Generally, an eddy current probe utilizes a miniature transformer having two coils mounted in close concentricity with one another. By bringing the probe into proximity with a conductive metallic specimen while exciting the primary coil thereof with an alternating current, the mutual inductance of the pair of coils is altered. This results in induction of eddy currents in the specimen by the magnetic fields of the probe coils. The strength of the eddy current field is deduced from a change in the reactance of the probe. The eddy current activity in the specimen film is inversely proportional to the distance (lift-off) between the coils and the conductive specimen. If the "lift-off" is a constant the measured reactance of the probe system is a function of the thickness, magnetic permeability, electrical conductivity, and density of the metal specimen, as well as some other metallurgical texture characteristics of the specimen. Various approaches for measuring one of these parameters while holding the others constant has resulted in a number of commercial instruments. However, all of these approaches are deficient because the reactance measurement is compromised by "lift-off" phenomena. In most situations, all of the other factors can be adequately controlled except lift-off. Accordingly, precise thickness measurements are not possible since probe proximity and presentation attitudes are not precisely controlled.
Most commercial probes are hand held, pencil-like devices with coils mounted in the free ends, thereof. Since these probes are manually pressed against the surface of the specimen, it is difficult to control pressure and coil orientation making thickness measurements in the ten micrometers regime quite unreliable.
There are several inexpensive devices in use which are adequate for low precision work. For example, a concentric, non-conducting bushing installed over a typical pencil probe provides it with a larger attitude orienting contact face. Crude wheeled vehicles are available for transporting the probes at a "constant" attitude and stand-off distance with respect to the surface of the specimen being tested. There is a device available from the Helmut Fischer Instrument Company in which transducers are spring loaded and presented to the surface of a specimen with a small arbor press so as to better control pressure and orientation. The makers of this device claims that it is possible to measure highly replicated average thicknesses in the range of +/-0.05 mm from a reference line. However, there are indications that the precisions of a single measurement could approach +/-0.01 mm on PVD aluminum and copper if the difficulties resulting from lift-off could be eliminated.
The patent literature includes a number of references that suggest floating an instrument with air pressure; e.g., U.S. Pat. Nos. 3,855,524 to Crawford; 3,884,076 to Studer; 4,450,404 to Williams et al.; and 4,528,507 to Domin et al. In each of these patents, the lift-off of the probe from the specimen is gravity opposed. The probe itself is lifted by the same column or film of air which is used to space the probe from the specimen. Accordingly, the devices are somewhat unstable in that stand-off of the probes is a function of a single air pressure source which must be sufficiently large to overcome the force of gravity on the total weight of the levitated device. This economy of regulation preempts a delicate adjustment of lift-off.
In view of the deficiencies of prior art approaches, it is necessary to provide methods of and apparatus for levitating a probe wherein the attitude of the probe is highly reproducible with little pitch or yaw during presentation. Moreover, it is desirable to have a probe which adjusts and maintains a preselected stand-off distance to within +/-20 millionths of an inch for the transducer of the probe. It may be necessary to accomplish the aforementioned without mechanically contacting the surface of the specimen so as to avoid physical damage thereto.
Finally it is desirable to move the work relative to the probe such that the probe hovers over the work at a fixed lift-off while continuous readings of the reactance are read--i.e., a scanning eddy current probe.